An integrated research program is planned to investigate the relationship of malignancy and differentiation and the functions of the protease plasminogen activator in normal neural development and in the malignant phenotype. The main focus for the last year has been on inducible differentiation of neuroblastoma-glioma hybrid 108 cells. Cells were grown in serum-free medium, and the choice between proliferation and differentiation was quantitated by daily observation of individual cells. The results of the analysis support the following model. N2 medium contains all the molecular components necessary to support either proliferation or differentiation. The choice between these two events appears to be stochastic. However, once a cell begins on the differentiation pathway, it exhibits persistence, which is the tendency of cells with long neurites to continue neurite growth, to re-extend neurites following division in N2 medium, and to become refractory to re-entering the proliferative pool upon challenge in serum-containing medium. Persistence is, however, extinguished following one round of division in serum-containing medium. Addition of 0.5 to 1.0 mM dibutyryl cyclic AMP to N2 medium resulted in an increased probability to form neurites and a decreased probability to divide. Thus, the malignant phenotype may be modified by compounds which affect the probabilities of these divergent behaviors. The serum challenge experiment indicated that length of the refractory period prior to cell division was proportional to the length of time the cells were exposed to inducing conditions. However, even cells of highly differentiated morphologies eventually divided, indicating that they were not committed to terminal differentiation. Future experiments will attempt to induce terminal differentiation in malignant neuroblastoma cells by modulating the initial probability to differentiate and by increasing the length of exposure to inducing conditions. (M)